This invention relates to a color phase correction circuit for a video disc playback device capable of preventing color irregularity in a picture caused by track-kicking in reproducing a picture in a still-picture mode or a trick-play mode.
Functions of a video playback device include still-picture and trick-play functions. A still-picture is produced by repeated reproduction of a picture from the same track. The trick-play function includes a frame skip function. These functions are obtained by track-kicking or track-jumping a laser beam of an optical pickup to next track. In the prior art video disc playback device, however, if a track-kick is executed, it will cause a color inversion or color irregularity in a scanning line, bordering where the track-kick is executed. This is because, in a CAV disc of NTSC system whose revolution speed is constant, there is 180 degree difference in color burst between adjacent tracks.
For example, as shown in FIG. 2, in a case where a track-kick is executed during a horizontal scanning period, the color of a picture during the period 1 after the track-kick is inverted by the track-kick, because the color of the picture on the side of a TV receiver is determined by the color burst A of the preceding track. At the next scanning line, a reference sub-carrier wave on the side of the TV receiver is re-established by the color burst B so that this color inversion is cancelled during the period 2. However, it takes relatively long time for this re-establishment due to the change of large magnitude, i.e., 180 degrees, of the color burst, so that irregularity of the reference sub-carrier tends to occur during the re-establishment thereby causing disturbance in the color of the picture.
In the foregoing manner, if a track-kick was executed in the prior art video disc playback device, it always caused a color inversion in one of the scanning lines, which was conspicuous in the picture, or produced a color irregularity in the picture for a while after the track-kick.
To cope with this problem, it has been practiced to prevent the color inversion by utilizing the time base control of a video signal by TBC (Time Base Corrector), i.e., by forcibly varying, in track-kicking, the time base of a video signal forward or backward by a time length which corresponds to 180 degree phase of a color burst.
FIG. 3 shows a prior art circuit to realize the above described method. A signal reproduced from a disc is applied to a main TBC 10. The main TBC 10 removes jitter (time base variation) in a video reproduced signal, and is constructed, for example, of a continuously variable delay circuit of binaly signal. For such continuously variable delay circuit, a CMOS gate circuit as disclosed in Japanese Preliminary Patent Publication No. 61-39721, for example, may be employed. The CMOS gate circuit is constructed of a P-channel MOS-FET and an N-channel MOS-FET. In this circuit, charging and discharging currents flow in accordance with load capacity formed on the output side to provide a delay characteristic. The delay time of this circuit changes in accordance with the power voltage, because conductance of the elements changes in accordance with the power voltage. That is, the high the power voltage, the shorter is the delay time, and the lower the power voltage, the longer the delay time. The continuously variable delay circuit consisting of the CMOS gate circuit delays the binaly signals by utilizing this delay characteristic.
The main TBC 10, being controlled in its delay time by a delay amount control circuit 12 as will be described later, absorbs jitter in the reproduced signal from the disc.
The reproduced signal delivered out of the main TBC 10 is applied to a BPF (band-pass filter) 14, in which a video signal component is separated, and a horizontal synchronizing signal is detected through an FM detection circuit 16 and a horizontal synchronizing signal separation circuit 18. From a reference counter 20, a saw-tooth wave generation timing signal TGRF is produced at a timing which synchronizes with a reference horizontal synchronizing signal. This signal TGRF is received by a saw-tooth wave generation circuit 22 which thereupon generates a saw-tooth wave TGEIN of a predetermined gradient to detect a phase error.
A sample-hold circuit 24 holds TGEOUT which is a level corresponding to the phase error between the reproduced horizontal synchronizing signal and the reference horizontal synchronizing signal, by sample-holding, with the detected horizontal synchronizing signal, the level of the saw-tooth wave TGEIN produced by the saw-tooth wave generation circuit 22 in synchronization with the reference horizontal syncronizing signal. The delay amount control circuit 12 controls the time base of the disc reproduced signal, by controlling the delay time of the main TBC so as to reduce this phase error to zero.
The video signal component separated in the BPF 14 is applied to a color TBC 26. The color TBC 26, which removes jitters which can not be removed in the main TBC 10, may be constructed, for example, of a continuously variable delay circuit of binary signal employing a CMOS gate circuit as in the main TBC 10. A delay amount control circuit 28 compares in phase a color burst in a video signal separate through an FM detection circuit 30 and a color subcarrier BPF 32 located on the output side of the color TBC 26, with a reference clock of 3.58 MHz, which corresponds to a regular subcarrier produced by the reference counter 20, and controls, in case it is constructed of a CMOS gate circuit, the power voltage of the color TBC 26 in accordance with this phase error to control the delay time and absorb a fine jitter in the video signal. In this manner, a video signal in which jitter has been removed is provided from the color TBC 26.
A system control 34 produces various commands. When a track-kick command for one track is produced in accordance with a still-picture command or a trick play command, a phase inversion information signal CBPCH is provided by a phase inversion information output circuit 36. This signal CBPCH is a signal which changes alternately to "1" and "0" each time a track-kick command is provided.
The signal CBPCH is reduced in its voltage level by an attenuator 38, to a level whose level "1" changes the delay time of the main TBC 10 by time length (140 nsec) which, correspondes to the 180 degree phase of the color burst, and becomes a signal CBPCH'. When the signal CBPCH' is delivered out, the delay amount control circuit 12 controls the delay time of the main TBC by adding this signal CBPCH' to the phase error output TGEOUT provided from the sample-hold circuit 24. The main TBC 10 is thereby controlled in delay time by time length corresponding to the 180 degree phase forward or backward at each track-kick. As a result, the color inversion caused by a track-kick is prevented.
FIG. 4 shows an operation of the circuit of FIG. 3 in track-kicking. The saw-tooth wave TGEIN is generated in a predetermined gradient with its timing of generation (charging) and reset (discharging) being determined by the saw-tooth wave generation timing signal TGRF which is synchronized with the reference horizontal synchronizing signal. This saw-tooth wave TGEIN is sample-held by the sample-hold circuit 24 at the timing of the reproduced horizontal synchronizing signal, and this sample-hold value becomes the phase error output TGEOUT.
The phase inversion information CBPCH repeats "1" and "0" each time a kick pulse is applied, and voltage of this "1" level is reduced by the attenuator 38 to the level which corresponds to the delay time of 140 nsec to become the signal CBPCH', which is added to the sample-hold output. The sum signal becomes the output of the delay amount control circuit 12. In this way, the color inversion of a picture is prevented by shifting the time base of the reproduced signal from the disc forward or backward by time length corresponding to the 180 degree phase of the color burst.
In this prior art circuit, the voltage CBPCH' corrected by a track-kick was added in an analog manner to the phase error voltage TGEOUT obtained by sample-holding the saw-tooth wave TGEIN with the reproduced horizontal synchronizing signal. In this circuit however, it was difficult to set the constant of the attenuator 38 to generate the voltage CBPCH' which is voltage corresponding to the delay time corresponding to the 180 degree phase of color burst. Therefore it was difficult to deliver out an accurate voltage and thereby prevent the color irregularity accurately.
It is, therefore, an object of the invention to provide a color phase correction circuit, capable of preventing a color inversion caused by a track-kick by detecting an accurate delay amount which corresponds to the 180 degree phase of the color burst.